Various methods have been proposed in the art and some progress has been made in safening urea formaldehyde(UF)-wood composite products by reducing the flammability of and the emission of formaldehyde vapors and odors from the composite products. However, the fire retardants used in previous methods undesirably reduce the strength of the finished product and increase the hazard of toxicity to humans. Further, the formaldehyde scavengers used in prior methods undesirably reduce the chemical stability of the resins used in the composite products and allow for slow hydrolysis and chronic formaldehyde odors during the long term service of the finished products. None of the known prior methods serve to safen UF-wood composite products by reducing both the flammability and formaldehyde vapor emission while substantially retaining the physical strength of the composites.
Where toxicology and miscibility with hydrophilic wood resins are not concerns, halogenated organic compounds, such as polybrominated bisphenol or polychlorinated phosphates can be used as strong fire retardant agents in the preparation of wood composite products. However, many composite UF-wood products, such as particle board, fiber board-and plywood are used around people where appreciable toxic hazards are not acceptable and therefore the use of toxic fire retardant materials is precluded. Moreover, fiber board, particle board, plywood and other wood-based composite products comprise UF resins and hydrophilic materials which are often not miscible with many polyhalogenated organic fire retardants. Low cost hydrophilic, inorganic, fire retardants, such as ammonium sulfate and ammonium phosphate, which are economically effective and reasonably safe in the environment as evident by their use as fire retardants in fighting wild brush and forest fires, are commonly used as a result. These materials, however, do not combine with UF polymers, but rather convert to soluble nitrates and therefore when used alone, are of little long term value as fire retarding agents in UF-wood composite products.
Boron compounds, in particular boric acid and sodium orate (borax), are widely used as fire retarding agents in cellulosic insulation and particle board. Sodium borate is not as effective as boric acid and its use results in a substantial reduction in the physical strength of the fiber boards and particle boards containing fire retarding amounts of sodium borate. Boric acid, on the other hand, acts as an acid catalyst and prematurely sets the UF resin which is used to cement the wood pieces in the UF-wood composites. Additionally, the catalytic action of boric acid continues in the finished composite product and eventually serves to degrade the UF resin bond strength and weaken the composite after unacceptably short service lives.
The emission of formaldehyde vapors and odors from wood composite products, such as particle board and plywood containing UF resins, have been a matter of concern to toxicologists and have been subject to regulation by government agencies, such as Housing And Urban Development (HUD), which are concerned with toxic emission in industrial and residential environments. The requirement of reduced formaldehyde concentrations and vaporizations from UF-wood composite products has been met by various processes which add unreacted urea to the UF resin used to bind the composite materials and, in some cases, to the wood particles themselves. In the book entitled Urea-Formaldehyde Resins by Meyer (Addison-Wesley Publishing Company, 1979), it is stated at page 233 that "Today, resins contain far less free formaldehyde. Thus, initial odor is greatly reduced. Ironically, the reduction of initial urea/formaldehyde ratio (required) to reduce initial (formaldehyde) odor produces chemically less stable resins, more susceptible to slow hydrolysis and chronic odor."
Specific processes known in the art which involve the inclusion of melamine in a wood product, utilize melamine as a component of the cured resin at the time of incorporation with the wood of the product rather than in a substantially free form at the time it is combined with the UF resin and wood. For example, U.S. Pat. No. 4,510,278 discloses a method of preparing particle board using a urea-formaldehyde resin bonding agent. The bonding agent additionally contains melamine, either in a pure or condensate form. In the method disclosed for preparing the bonding agent, the melamine is added during the condensation of the urea and formaldehyde and, accordingly, is incorporated as a component of the resin and not maintained in substantially free form. The melamine, therefore, loses its individual properties and demonstrates the properties of the newly formed combined resin.
U.S. Pat. No. 4,536,245 discloses a curable melamine-urea-formaldehyde base resin usable as a binder, particularly in particle board. The base resin, however, is a condensate, i.e. the urea, formaldehyde and melamine are polymerized together to form the resin prior to their use as a binder.
U.S. Pat. No. 4,176,115 discloses a fire resistant construction material formed by curing a powdered blend of dry melamine-formaldehyde resin and an incombustible component. The melamine is used as a component of the resin. Melamine is not disclosed as being a suitable incombustible component. The incombustible component forms the major structural properties of the article formed and is generally a mineral, such as asbestos.
U.S. Pat. No. 4,117,200 discloses fire retardant wood products made from wood particle board, a glue, such as a UF resin, and a melamine phosphate-phosphoric acid which is present as a flame retardant. The '200 patent states that if the flame retardant is added before the glue or simultaneously with the glue that the retardant clumps and coats in a non-uniform manner.
U.S. Patent Nos. 3,939,107; 4,152,320; and 4,339,357 disclose fire retardant particle boards formed utilizing a resinous binding agent wherein melamine is a component of the resin. Non-resinous melamine is not present at any time.
The art does not disclose the addition of a substantially unreacted non-resinous melamine to a UF-wood composite to provide a finished composite product having reduced flammability and reduced formaldehyde vapor emission.